DR and BDR are only necessary in multi-access network, this is because multi-access network connects to many routers and hence each ospf speaking router will generate huge LSA traffic. Hence to solve this issue the DR scheme is created.

The role of the DR is to send LSA update to every ospf speaking routers, and those DRother routers (neither DR nor BDR) will send LSA update to the DR, LSA hello is still sent by all ospf routers for keepalive purpose. Also DRother routers will only form adjacency to DR and BDR. BDR is the backup of DR.

In point-to-point and point-to-multipoint networks, no DR is required hence BDR is also not necessary. Because point to point network has a pair of routers, hence ospf speaking routers formed adjacency with each other via unicast.

The default hello interval for broadcast network is 10s and 30s for non-broadcast network. This is important because routers will not form adjacency with each other if hello interval does not match.

Area 0 network is a default non-broadcast multi-access (NBMA) network, in this network ospf adjacency has to be formed manually using the neighbor command.

In MA network a DR is needed. Since R1 is the hub with full connectivity with R2 and R3 R1 should be elected as DR, however because the router-id of R1 is the lowest among R2 and R3, R3 will probably be DR and R2 be a BDR if there is no user intervention. R2 and R3 are spoke routers, they do not have full connectivity with each other. If R3 needs to get to R2, R3 will have to pass through R1 first, R2 has the same situation too.

Hence to prevent R2 and R3 from being DR and BDR ip ospf priority 0 command is used. The highest priority value wins the DR, the next highest will be the BDR, 0 indicates that router can neither be elected as DR nor BDR.

this is old post for anyone reading this blog in area 0 as the network is non broadcast and we are configuring static neighbor it is not necessary to put broadcast in frame relay map ip x.x.x.x broadcast

Rakesh, I removed the broadcast option in my lab and the neighbor came down. (“frame-relay map ip x.x.x.x yyy” instead of “frame-relay map ip x.x.x.x yyy broadcast”) I think the broadcast option is taking the broadcast messages and making them unicast so they will work on the non broadcast network.

Rakesh, My lab was not using the neighbor command. It was using a broadcast network type on R1 along with the broadcast option in the frame map command. You are right, broadcast isn’t needed with the above setup.